1-6. TYPES OF NEURONS

Neurons in the body can be classified according to structure and function. Classification by structure is based on the number of processes (projections or protrusions) extending from the cell body. Neurons are classified by functions according to the direction in which the neurons transmit impulses.

a. Classification of Neurons by Structure. Classifications are as follows: multipolar neurons, bipolar neurons, and unipolar neurons. Multipolar neurons have one axon and several dendrites. Brain and spinal cord neurons are generally multipolar neurons. Bipolar neurons have one axon and one dendrite. Most of these neurons are found in the retina of the eye, the inner ear, and the olfactory area. Neurons with only one process extending from the cell body are termed unipolar neurons. The one process divides with one part acting as an axon and the other part functioning as a dendrite. Unipolar neurons are found in the posterior (sensory) root ganglia of the spinal nerves.

b. Classification of Neurons by Function. According to function, there are two types of neurons: sensory neurons (also called afferent neurons) and motor neurons (also called efferent neurons). Sensory neurons conduct impulses from the receptors in the skin, sense organs, and viscera (the large internal organs) to the brain and the spinal cord. These neurons conduct impulses from receptors to the central nervous system and are usually unipolar. Impulses transmitted from the brain and spinal cord to either muscles or glands are carried by motor neurons. These neurons conduct information away from the central nervous system to the skin, muscles, glands, and organs of the body.